Mixed powder for powder metallurgy and process for producing same

Abstract

A process for producing a mixed powder for powder metallurgy in which graphite segregation can be prevented and which has satisfactory flowability and brings about satisfactory lubricating properties, the process comprising: selecting an organic binder which, when the solubility of an organic lubricant in a given organic solvent at a given temperature is taken as 1, has a solubility in the same solvent at the same temperature of 2 or higher; mixing the organic lubricant and the organic binder with the given organic solvent together with an iron powder to prepare an iron-powder slurry in which the organic lubricant and the organic binder have been dissolved in the organic solvent; and removing the organic solvent from the iron-powder slurry by vaporization to precipitate the organic lubricant and the organic binder in this order.

Description

[0001]This application is a 371 of PCT / JP11 / 76168, filed Nov. 14, 2011.TECHNICAL FIELD[0002]The present invention relates to a powder metallurgy technology for producing a sintered body by molding and sintering an iron-base powder, in particular to a mixed powder for powder metallurgy which can suppress the segregation and dust emission of graphite and has both the flowability and lubricity of the mixed powder.BACKGROUND ART[0003]In powder metallurgy for producing a sintered body by using an iron powder or a copper powder as the main raw material, generally a mixed powder containing the powder of a main raw material, an auxiliary material powder (a graphite powder, an alloy component, etc.) for improving the physical property of the sintered body, a lubricant, and others is used. In order to improve the mechanical properties (strength, hardness, etc.) of a sintered body in particular, generally a means of adding a carbon supply component (carbon source) such as graphite, molding a m...

AI Technical Summary

Benefits of technology

[0023]A production process according to the present invention makes it possible: to obtain a mixed powder for powder metallurgy wherein an iron powder is covered with an organic lubricant and an organic binder; and to give both flowability and lubricity to the mixed powder for powder metallurgy. Further, when graphite is used in a production process according to the present invention, it is possible to prevent the graphite from segregating.

Problems solved by technology

Since graphite has a smaller specific gravity and a smaller grain size than an iron powder however, a problem is that, only by mixing them, the graphite is largely separated from the iron powder, the graphite segregates, and they cannot be mixed homogeneously.

As a result, a part having a high carbon concentration is formed in a sintered body, a cementite structure precipitates there, and mechanical properties deteriorate.

If a carbon content varies by the segregation of graphite in a sintered body, parts having a stable quality can hardly be produced.

Further an arising problem in a mixing process and a molding process is that the segregated graphite powder causes dust emission, and the deterioration of a work environment and the lowering of the handleability of a mixed powder are caused.

The method has an advantage that a mixed powder can be produced with simple equipment but a problem of the method is that, if a liquid additive of a quantity necessary for exhibiting a segregation prevention effect is added, a liquid bridge force acts among iron particles and flowability deteriorates extremely.

The method has the advantages that graphite can stick without fail and there are many choices in adopting a lubricant used but the flowability of a mixed powder may be insufficient depending on some quantities or some types.

The drawback of the method is that the temperature control during mixing is very important for uniformly sticking the melted lubricant onto the surface of the iron powder and the number of choices for a usable lubricant is restricted.

That is, if the flowability of a mixed powder is inferior, the arising problems are that bridging is caused at the upper part of the outlet in a hopper, thus the discharge is hindered, and a hose clogs between the hopper and a shoebox.

Further, in the case of a mixed powder having a poor flowability, even when the mixed powder is outpoured forcibly from a hose, a mold, particularly a part of thin-wall, is not filled and a sound molded body may not be produced in some cases.

If the quantity of the added lubricant decreases however, the lubricity lowers considerably as a matter of course, the friction coefficient between a molded body and a mold face increases when the molded body is extracted from a mold, and that causes die seizure and mold damage.

Consequently, it has been difficult to obtain both lubricity and flowability simultaneously.

Further, it is difficult to obtain both lubricity and flowability simultaneously also from the viewpoint of the type and melting point of a lubricant.

That is, stearic acid or stearic acid amide generally having a low melting point is excellent in lubricity but, in a lubricant having such a low melting point, aggregation is caused and flowability deteriorates in some cases.

When an ambient temperature is high in particular, the drawback appears conspicuously.

In contrast, metallic soap or ethylene bis-amide having a high melting point can maintain a good flowability even when an ambient temperature rises but lubricity is inferior to the stearic acid amide or the like having a low melting point.

In this way, in consideration of the quantity and type of a lubricant added, to materialize a mixed powder having both lubricity and flowability simultaneously has been a long-term challenge.

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